In various paper fiber treatment systems it is desirable to separate from the papermaking fibers, contaminants which lower the quality of the fiber. In a conventional fiber treatment system in a paper making operation, for example, the rejects from the primary screening operation are often directed to a so called tailing screen, which separates acceptable fiber ejected from the primary screen with the rejects and directs the fibers reclaimed back into the fiber treatment system while the rejects from the tailing screen are rejected from the system.
Another, less conventional example of a paper fiber treatment system is shown in U.S. Pat. No. 3,736,223, in which solid waste materials, such as typical municipal refuse which contains a high percentage of fibrous materials, are deposited in a pulper tub with a liquid, such as water, and subjected to hydraulic and mechanical shear forces to reduce the frangible solids to a predetermined size before they are extracted in slurry form from the pulper tub together with a portion of the liquid.
The slurry of liquid and frangible solids is thereafter processed through what may be broadly termed screening apparatus to improve the quality of the paper fibers contained in the slurry and separate out undesirable, non-fibrous contaminants. The accepts from the screening apparatus are further processed and the rejects are directed to a tailing screen. There the majority of the solids other than fibers is rejected from the system and the remaining accepts are processed further.
Still another example of a paper fiber treatment system is the digesting operation in a paper making operation, wherein wood chips and cooking liquor are deposited in a digester and the chips cooked with the liquor at elevated temperatures and pressures for some predetermined period of time to break down the wood chips and enable free fibers to be obtained from the chips for further processing. After the digesting operation there will still remain some undefibered solids, such as knots, which must be removed or separated from the acceptable fibers. Therefore, in conventional pulp preparation systems the digesters will be followed by a screening operation to remove undesirable contaminants such as knots.
It will be seen, therefore, that generally tailing screens are used to separate and discharge from the system rejectable material while acceptable material is recirculated and processed further, and, of course, it is important that the rejects be free or almost free of acceptable material. The most common type of screening apparatus used for this purpose is a vibratory screen.
Such screens, however, have many, art recognized undesirable characteristics. For example, the vibrating screens, by their very nature, are noisy and dirty in that they throw off liquids and, to some extent solids, during the screening operation and are generally unpleasant to work around. Additionally, the vibrating screens require special mounting and are susceptible to mechanical failure due to the inertial forces imposed on the apparatus as it vibrates during normal operation.
Screw type thickeners have been used for dewatering in various types of operations. For example, U.S. Pat. No. 3,188,942 shows a screw type thickener for dewatering fibrous material, such as waste material, after it has been disintegrated. Also of interest is U.S. Pat. No. 3,616,932 which discloses a thickener for separating pulp stock and water. An additional patent of interest is U.S. Pat. No. 3,080,065, which again utilizes a screw type thickener for separating a caustic neutralizing solution utilized to remove bottle labels from the labels themselves. A further example in U.S. Pat. No. 2,471,517 which discloses a machine for separating spent hops from wort.
In conventional screw thickeners, the goal generally is to separate as much liquid as possible from the solids carried in suspension in the liquid. In accordance with this goal the perforations in the casing in which the screw revolves are kept as small as possible to minimize the passage of solids through the perforations in the casing. In paper fiber thickening operations this reduces as much as possible the loss of fibers rejected from the thickener with the liquid. In operations where the liquid is valuable, it reduces as much as possible the entrainment of undesirable solids with the liquid.
As a practical matter the minimum size of the perforations in the casing is limited by manufacturing considerations, plate thickness and the required throughput rate for the thickener.